Add development requirements (requirements.dev.txt)

To be able to run the tests of datapackage developers are going to
have to install two different packages:

* nose (nosetests)
* mock (mock library for tests)

These have been added to a requirements.dev.txt file. Note that
this is not the conventional requirements.txt file, since these
are only requirements for those who hack on datapackage.
Conform to pep8 and other checkers

This adds flake8 as a checker to requirements.dev.txt. flake8 is
a wrapper around pep8, pyflakes and mccabe which can be run
against the source code to check for pep8 errors/warnings and
other errors in the source file (like unused imports).

After running flake8 against the code base a few errors have been
found and are corrected.
Add contribution guidelines to the project

Since we're now heading for greatness thanks to Jessica we need to
get ourselves some contribution guidelines so that we can all work
well together without any conflicts (by explaining things like
tests and that we use the development branch etc.)

Also, we're a happy team! Woooo!